1. Field of the Invention
The present invention relates generally to amplifier circuits, and specifically, to high performance amplifier circuits.
2. Background Information
Operational amplifiers of various types are well known in the prior art. Such amplifiers may be characterized as having a differential input of a high impedance, a single-ended output of a low impedance and a high gain, as are generally illustrated in FIG. 1. Most present day operational amplifiers are required to operate from a single power supply voltage and have an output voltage that is capable of swinging to both supply rails. These amplifiers typically have an input stage that converts the differential input to a single-ended drive for the complementary output drivers. Such an amplifier may be seen in FIG. 2, which is representative of prior art amplifiers. In the circuit of FIG. 2, the single-ended output of the differential input stage drives the N channel output driver N1 directly, while P channel device mirrors the current of current source I1 to the P channel output driver P1. The current source I1 may be an independent current source, representative of class A operation, or as is more common in present day amplifiers, a controlled current source whose current is inversely dependent on the current in output driver N1, and hence is representative of class AB operation. Such an implementation of a class AB amplifier that is representative of prior art amplifiers is depicted in FIG. 3. In the circuit of FIG. 3, device N2 has its gate connected to the gate of device N1, and hence its drain current mirrors that of device N1. This current flows into the emitter of transistor Q2 and sets the base-emitter voltage of Q2 accordingly. Since the sum of the base-emitter voltages of transistors Q1 and Q2 is set by the independent current source, I.sub.ref, flowing through transistors Q3 and Q4, and is therefore constant, the base-emitter voltage of transistor Q1, and hence its collector current, will decrease as the current in device N2 and transistor Q2 increases, thus fulfilling the requirements for class AB operation.
Furthermore, there are many circuit applications that require analog outputs that are minimally degraded by noise on the power supplies. Examples of these applications include high quality signal processing, signal conditioning, laptop/notebook computers, cell phones and portable headphone speaker drivers. These applications require amplifiers that exist on the same printed circuit board (PCB) with other high speed/high power circuitry and, often, very noisy switch-mode power supplies. These high speed/high power circuits tend to corrupt the power supply voltage, resulting in poor circuit performance. A highly regulated power supply may reduce the effect of noise, but the headroom voltage which is demanded by the regulator is lost for the amplifier and therefore, the output only has a rail-to-rail swing less the headroom loss in the regulator.
Traditionally, an amplifier can either have a good drive capability, i.e., a full rail-to-rail swing but a poor supply rejection or a good supply rejection but poor drive capability. Therefore, there is a need to have an amplifier circuit that has good supply rejection and full rail-to-rail output range without incurring hardware complexity.